The Rhox gene cluster suppresses germline LINE1 transposition

Transposable elements (TEs) are mobile sequences that engender widespread mutations and thus are a major hazard that must be silenced. The most abundant active class of TEs in mammalian genomes is long interspersed element class 1 (LINE1). Here, we report that LINE1 transposition is suppressed in the male germline by transcription factors encoded by a rapidly evolving X-linked homeobox gene cluster. LINE1 transposition is repressed by many members of this RHOX transcription factor family, including those with different patterns of expression during spermatogenesis. One family member--RHOX10--suppresses LINE1 transposition during fetal development in vivo when the germline would otherwise be susceptible to LINE1 activation because of epigenetic reprogramming. We provide evidence that RHOX10 suppresses LINE transposition by inducing Piwil2, which encodes a key component in the Piwi-interacting (pi) RNA pathway that protects against TEs. The ability of RHOX transcription factors to suppress LINE1 is conserved in humans, but is lost in RHOXF2 mutants from several infertile human patients, raising the possibility that loss of RHOXF2 causes human infertility by allowing uncontrolled LINE1 expression in the germline. Together, our results support a model in which the Rhox gene cluster is in an evolutionary arms race with TEs, resulting in expansion of the Rhox gene cluster to suppress TEs in different biological contexts. FACS-purified germ cells from fetal Rhox10-null;Oct4-eGFP+/+ (KO) and littermate Oct4-eGFP+/+ (control) mice were used for RNA-seq.转座子（Transposable elements, TEs）是可移动的遗传序列，可引发广泛的基因组突变，因此是一类亟需被沉默的主要危害因子。哺乳动物基因组中丰度最高的活性转座子类别为长散在核元件1类（long interspersed element class 1, LINE1）。本研究发现，雄性生殖系中的LINE1转座活动可被一个快速进化的X连锁同源框基因簇所编码的转录因子抑制。该RHOX转录因子家族的多个成员均可抑制LINE1转座，涵盖在精子发生过程中表达模式各异的家族成员。其中一个家族成员——RHOX10——可在体内胚胎发育阶段抑制LINE1转座，而此时生殖系因表观重编程而易受LINE1激活的影响。本研究提供证据表明，RHOX10可通过诱导Piwi样蛋白2（Piwil2）的表达来抑制LINE1转座；Piwil2是Piwi互作RNA（Piwi-interacting RNA, piRNA）通路的关键组分，该通路可抵御转座子的侵害。RHOX转录因子抑制LINE1的功能在人类中保守存在，但在数名不育人类患者的RHOXF2突变体中该功能丧失，这提示RHOXF2功能缺失可能通过使生殖系中LINE1的表达失控而引发人类不育。综上，本研究结果支持如下模型：Rhox基因簇与转座子处于进化军备竞赛之中，这推动了Rhox基因簇的扩张，使其能够在不同生物学情境下抑制转座子活性。本研究使用了来自胚胎期Rhox10敲除（Rhox10-null;Oct4-eGFP+/+，即KO组）以及同窝野生型Oct4-eGFP+/+（对照组）小鼠的荧光激活细胞分选（Fluorescence-Activated Cell Sorting, FACS）纯化生殖细胞进行RNA测序（RNA-seq）。